This invention relates to apparatus and a method for synchronizing various component signals of an augmented television signal so that they may be properly recombined at a receiver to reproduce an enhanced image.
A conventional television receiver, such as a receiver in accordance with NTSC broadcast standards adopted in the United States and elsewhere, has a 4:3 aspect ratio (the ratio of the width to the height of a displayed image). Recently, there has been interest in using higher aspect ratios for television receiver systems, such as 2:1, 16:9 or 5:3, since such higher aspect ratios more nearly approximate or equal the aspect ratio of the human eye than does the 4:3 aspect ratio of a conventional television receiver. Video images having a 5:3 aspect ratio have received particular attention since this ratio approximates that of motion picture film. However, widescreen television systems which simply transmit signals having an increased aspect ratio as compared to conventional systems are incompatible with conventional aspect ratio receivers. This makes widespread adoption of widescreen systems difficult.
It is therefore desirable to have a widescreen system which is compatible with conventional television receivers. One such system is disclosed in a copending U.S. patent application of C. H. Strolle et al., Ser. No. 078,150 now U.S. Pat. No. 4,816,899, "Widescreen Television System", filed July 27, 1987.
It is even more desirable to have such a compatible widescreen system with provisions for enhancing or extending the definition of the displayed image so as to provide extra image detail. For example, such widescreen EDTV (extended definition television) system may include apparatus for providing a progressively scanned image.
EDTV systems have been proposed which include provisions for inserting a signal representing the more detailed areas of an image into a portion of the video spectrum which is currently unused or which is not used efficiently under current television standards. One of these proposals is set forth in U.S. Pat. No. 4,660,072 issued in the name of T. Fukinuki on Apr. 21, 1987 and hereby incorporated by reference. A second proposal is presented in an article by Y. Yasumoto et al. "An Extended Definition Television System Using Quadrature Modulation of the Video Carrier with Inverse Nyquist Filter" IEEE Transactions on Consumer Electronics, August, 1987, PP 173-180, which is hereby incorporated by reference. A third proposal is presented in an article by M. A. Isnardi et al. entitled "Decoding Issues in the ACTV System", IEEE Transactions on Consumer Electronics, February 1988, pp 111-120, which is hereby incorporated by reference.
In the Fukinuki proposal, relatively high frequency information is separated from an original high definition video signal, frequency converted to occupy a lower band of frequencies and then modulated onto an alternate subcarrier signal which is within the frequency spectrum of a conventional video signal. This alternate subcarrier signal, like the chrominance subcarrier signal of a conventional composite video signal, is an odd multiple of one-half the horizontal line frequency but, unlike the chrominance subcarrier signal, it switches in phase by 180.degree. from field to field. This modulated alternate subcarrier signal may be substantially recovered at the receiver.
In the receiver, the high resolution signal is recovered from the modulated alternate subcarrier, restored to its original band of frequencies and added to the conventionally decoded luminance signal to produce a high definition luminance signal.
The Yasumoto et al. proposal uses an enhancement signal which may represent either high frequency luminance information or side-panel information needed to expand the aspect ratio of a transmitted image from 4:3 to, for example, 5:3. This enhancement signal is frequency converted to occupy a lower band of frequencies and then modulated onto a carrier which is in quadrature phase relative to the picture carrier signal of the transmitted video signal.
At the receiver, this enhancement signal is recovered by synchronous demodulation of the video signal. The recovered signal is then frequency converted to occupy its original band of frequencies and combined with the conventionally processed video signal to produce an enhanced (high definition or widescreen) video image.
The Isnardi et al. proposal compresses side panel low frequency information of a widescreen image into the horizontal overscan region of a compatible NTSC Television signal. The high frequency information for the side panels and high frequency information for the entire widescreen image are quadrature modulated onto an interlaced carrier which changes phase by 180.degree. from field to field. A fourth signal component, which is used to reconstruct a progressive scan image from the interlace scan signal, is modulated in quadrature with the combined first three components onto the video carrier signal.
At the receiver, a combination of synchronous demodulation and intra-frame processing is used to recover the four components. These components are combined to produce an extended definition television signal.
In any of the systems described above, slight timing errors may occur in the signals used for frequency-converting the enhancement signals at the transmitter and at the receiver, or in signals used to determine relative pixel locations of the side panel and center panel information for the extended definition and/or enhanced aspect ratio image. These timing errors may distort the reproduced enhanced image causing, for example, dark areas of detailed information to appear erroneously bright or causing visible seams where the side panel and center panel regions of a wide aspect ratio image are joined.